Well used brass - expander mandrel & turn?

[bax]

I have some well used brass - 308 Lapua and Lake City LR and 300 Win Mag Navy WW and Lapua. I did not turn the necks. After many firings, the neck thickness is not consistent from the top to the bottom of the neck. When I seat, bullets start smoothly then I push past a "ring" where the neck is thicker then the bullet slides with less friction.

It seems to me that the bullet is not supported by the whole neck but mostly by this ring. The parts of the bullet cylinder above and below the ring are not actually in contact with the bullet. Since the necks are not consistent from top to bottom, as a round feeds from the magazine the bullet could become slightly canted and this might be bad.

It just occurred to me that I could test this theory by shooting groups at 500 - some from the magazine and some hand-fed. Never mind. The question continues ...

As an associate member of the "hand loaders with OCD" group, I am thinking that maybe I want to fix this. So I came up with a plan. Suppose that I start by annealing then size my brass. That makes the neck outside diameter the same from the top of the neck to the bottom and it pushes the ring into the ID. Then I screw in a neck expander mandrel and run the necks over that. This makes the ID the same (more or less) and pushes the ring to the OD. Then I turn the necks thus cutting off the ring. The cut would leave the neck thickness as large as possible - more brass means more support.

1. do you know - or think that you know - that the ring is a problem? For what it's worth, I am not yet convinced that it is.

2. will this procedure actually cut off the excess brass that makes the ring? For example, maybe the mandrel temporarily pushes the ring but then it springs back ... or something.

3. aside from the time and cost it takes to do it, is there any downside you can see? Except for custom mandrels I have all the tooling I need to do this.

4. do you think that I will be able to see results on the target? Will it generally reduce group size or eliminate flyers? Both? I am inclined to think groups will remain the same but I will shoot fewer flyers.

5. suppose that I don't do this. If I start with new brass, is there a procedure that actually prevents the ring?

6. suppose that it helps, will the benefit be close in but not downrange? Downrange past 800 but not noticeable under 500?

7. screw this, yer wasting yer time ya moron, just go and shoot?

 

[bax]

I shoot brass until I get incipient head separations so this brass has been resized many times. Also, I am sizing for field conditions so brass shoulders contact the chamber when the bolt is most of the way closed -- or sometimes -.003 or -.004 just in case of dirt.

It seems to me that during the firing cycle, brass works like this. As I go down the list, time is increasing.

1. firing pin pushes cartridge until brass shoulder pushes against chamber shoulder

2. firing pin pushes firing pin cup and material against the anvil, the primer fires

3. case expands radially, gripping the chamber wall

4. case expands along its long axis, the base is pushed against the bolt face, thus stretching and thinning the brass just above the web area, brass elastic limits may be exceeded

5. under pressure from expanding powder gas, the receiver lugs are pushed back elastically, brass above web stretched and thinned more, brass elastic limits may be exceeded

6. pressure subsides, receiver un-stretches, brass un-stretches.

If brass elastic limits were exceeded above web, brass final length equal to chamber final size plus a small bit. If brass elastic limits were exceeded in the brass body section clamped to the chamber wall near the neck, some brass migrates from the body/shoulder into the neck. This latter effect is greater with longer cases - like my 300 Win Mag.

It is stretching in the brass body section that I am thinking creates the "dreaded donut". It takes several firing cycles to get donuts but I have a lot of brass that has been shot a lot of times and some of it has donuts.

Problem with Wilson inside neck reamer - post-sizing I must have necks that match their reamer OD. That is not always easy to get because the die and the brass thickness have to match up to the cutter. Maybe a custom mandrel is a way to do that.

I have a KM neck turning tool but I was not aware of this cutter on the mandrel - I think it is a new feature. I will check that out. Thanks!

 

[918v]

Not a new feature, been around for years. Scars the inside of the neck. Not the ideal way to do this.

What you’re contemplating will take a lot of work and will not give you the results you are looking for. A better way is to use American brass which is thinner in the shoulder area so it is less likely to form donuts. If you use a Lee collett neck die, you’ll never get donuts because the collett will squish whatever starts forming at the base of the neck and force it into the neck wall.

 

[spife7980]

You have it right, you can ignore the donut or you can turn it off.

You size down, use the expander mandrel that opens the neck to .001 beneath caliber but, as you said, spring back however when you turn you use a turning mandrel and that should mitigate any spring back.

You don’t have to trim all of it down, you can just turn the thicker part off if you want, or you can clean them entirely.

I wouldn’t inside ream and scratch them up.

 

[bax]

Not a new feature, been around for years. Scars the inside of the neck. Not the ideal way to do this.

What you’re contemplating will take a lot of work and will not give you the results you are looking for. A better way is to use American brass which is thinner in the shoulder area so it is less likely to form donuts. If you use a Lee collett neck die, you’ll never get donuts because the collett will squish whatever starts forming at the base of the neck and force it into the neck wall.

I think that lake city is american brass. Was I misinformed?

"...force it into the neck wall ..." can you explain?

 

[918v]

I was talking about Lapua.

The collet die squeezes the neck around a mandrel. There is so much force the brass extrudes. Whatever donut forms at the base of the neck is mashed by the collet against the mandrel and becomes part of the neck wall as brass flows forward.

 

[bax]

OK, I will try inside reaming.